Water Vapor #1

I notice that some posters have been discussing water vapor feedbacks and thought that I’d chip in a little. One aspect of water vapor’s role in the climate system that I find intriguing is absorption of near infrared (NIR) and visible solar radiation by water vapor. This is a topic of very active research by molecular spectroscopists, for several reasons, not least being the very significant discrepancy between observed atmospheric absorption of solar radiation and modeled absorption in the IPCC TAR models (up to 30 wm-2 as compared to 3 wm-2 for CO2 doubling).

Part of the recent interest was prompted by the discovery of a delicious clerical error under-transcribing the NIR absorptions in HITRAN-96 by an amount greater than the impact of 2xCO2. I’ll locate some references to this. New experimental work has shown that NIR absorption by water vapor was under-estimated by much more than even these clerical errors. Jonathan Tennyson, Professor of Physics, Head of Department, Department of Physics and Astronomy, University College, London and an extremely eminent near infrared spectroscopist, provided an interesting popular summary of some of these issues in 2003.

There are a number of popular misconceptions about the greenhouse effect, notably that it is a bad thing. On the contrary, the greenhouse effect is a significant factor in making the Earth habitable. Without it the average temperature on Earth would be lowered by about 30 K, which would make most of the planet’s surface decidedly chilly. Furthermore, it is the water vapour in the lower 10 km or so of the atmosphere, rather than man-made carbon-dioxide emissions, that contributes most to this warming effect.

But when the absorption values in the HITRAN database are used in model-atmosphere calculations, the results are disturbing. For clear skies, the models predict that the atmosphere absorbs much less sunlight than is measured by a variety of satellite and aircraft. The difference between the predictions and the measurements can be as large as 30 W m-2. (see "Radiation budget is called to account" by A Maurellis Physics World November 2001 pp22-23). This problem has become known as the absorption anomaly. And there are even worse problems in understanding absorption models when the sky is cloudy. Not all models underestimate the amount of atmospheric absorption because some physicists choose to add extra absorption to their models to mop up the surplus radiation. However, the physical cause of the missing clear-sky absorption and its exact wavelength distribution remain unresolved, and a source of fertile speculation. Everyone’s favourite molecule is always a candidate. The precise effect of these absorption bands is hard to determine, despite the best efforts of many talented and dedicated scientists…

Experiments that were performed by Roland Schermaul and the late Richard Learner at Imperial College in London in 2001 have cast previous measurements of the absorption spectrum of water into considerable doubt. The study was motivated by the European Space Agency (ESA), which was concerned that the uncertainty in water-vapour data was preventing important information on trace molecules in the atmosphere from being obtained. Schermaul and co-workers used the Molecular Spectroscopy Facility at the Rutherford Appleton Laboratory in the UK to study the absorption of light by water vapour in air at wavelengths that varied from the near infrared to the orange. They found that the strong spectral lines absorbed significantly more light – between 5% and 25% – than previous laboratory measurements had suggested. This conclusion was given partial support by first-principle quantum-mechanical calculations, which can be used to estimate the strength of these absorptions….

These measurements were put into atmospheric models by Joanna Haigh’s group at Imperial College to find out if they could explain the absorption anomaly. When the strength of the strong water absorption lines was increased in the model, the absorption of incoming sunlight rose by about 8 W m-2. This increased by a further 3 W m-2 when the weak line parameters that were measured by Schermaul and co-workers were included. Together these increases represent about half of the absorption anomaly. Unfortunately, however, the situation is not quite this straightforward.

There is a recent article (April 2005) by Martin Wild in GRL considering the handling of NIR absorption in recent GCMs. I’ll post up some info on this and some other NIR references.

15 Comments

I find this discussion very interesting, since I have done some work on calculating line absorption in the earth’s atmosphere in order to differentiate between stellar lines and telluric lines in spectra of stars obtained with ground based telescopes. I don’t recall having problems with the Hitran96 data base but I do remember that the AFCRL program I initially used to calculate the telluric spectrum didn’t give very good results in terms of agreement with observation. I ended up using a program written by a Canadian graduate student! In addition to
affecting the spectra of stars, the telluric lines do affect the measurement of stellar colours and occasionally, radial velocities measured from some stellar lines.

Steve,
Some what off topic. A comment by Peter Hartley got me to pick up Kramer’s “Physiology of Trees” 1960 to see what he knew on CO2 fertilization. I think this still stands. On page 84

“Carbon Dioxide. Photosynthesis of Temperate Zone trees that are well
exposed to light probably is inhibited most frequently by the low carbon
dioxide concentration of the air, which averages about 3 parts in 10,000,
or about 0.03 per cent by volume. During much of the time photosynthe-
sis varies directly with carbon dioxide concentration at levels present in
the field (Thomas and Hill, 1949), Decker (1947) found a linear rela-
tionship between photosynthesis and carbon dioxide concentration in the
range of 0.52 to 0.45 milligram per liter for both hardwoods and conifers
in the laboratory. The short-time rate of photosynthesis can be increased
by carbon dioxide concentrations up to ten times normal. However, either
such unusually high concentrations are toxic or other factors become
limiting (Frank and Loomis, 1949).”

and starting on the same page

“It often has been supposed that because volume percentage concen-
tration of carbon dioxide in air is essentially similar at different altitudes,
carbon dioxide has no influence in determining altitudinal species dis-
tribution. However, as Decker (1947) has pointed out, diffusion of carbon
dioxide into a leaf is a function of carbon dioxide pressure rather than
concentration, and the pressure of carbon dioxide in air varies directly
as total atmospheric pressure. Normal pressure of atmospheric car-
bon dioxide is approximately 0.228 millimeter of mercury at sea level
and only 0.130 millimeter of mercury at an altitude of 15,000 feet. This
altitudinal gradient in carbon dioxide pressure may well be one of the
complex of factors which govern altitudinal zonation of species.”

So could high altitude trees be even more sensitive to CO2 relative to temperature than most trees are? It would be a laugh. TREE PHYSIOLOGY 101 stuff right? This was known in 1947.

So if you take out all the errors you know about you no longer have a hockey stick but even if you assume CO2 constant before some year, you would have to chop the plot before CO2 fertilization kickes in. Perhaps RWs relative to MXDs show when CO2 fertilization starts to have an impact. Warning, I’m sheepskin free.

Hmm, a rare CO2 deficit about AD1425? Unrestrained speculation…

Sorry if this has been covered before but it would be a elemental flaw.

John, the reaction to increased CO2 levels is nonlinear and attenuates with higher concentrations. Since CO2 levels at high altitude are lower, the high-altitude trees are in a steeper portion of the curve. There is also considerable evidence that higher CO2 levels improve water use efficiency, sometimes dramarically. This would be made to order for high-altitude trees in ARID sites i.e. bristlecone pines (which compete with big sagebrush). We’ve got some comments on this in our E&E article. Steve

Re #2
John, this is just another elemental flaw in the AGW theory. The topic has been raised in other threads but your quotations spell it out very clearly. Not having a sheepskin leaves you free to ask real questions and look for real answers without risking the wrath of your peers if what you find differs from “The Consensus”. Thanks!

Steve, I found this quote in Delvin’s “Plant Physiology”, 1969 in a section on factors affecting the rate of photosynthesis, page 211. Also not quite on topic.

“In a fanciful way of thinking, the combustion of fuels may
be thought of as drawing on a savings account banked by
nature. During the Carboniferous Age, about 300,000,000
years ago, conditions for plant growth were the best they
ever were in the earth’s history. The world resembled a
large greenhouse of high humidity and high CO2 concentrations.
It has been suggested that the CO2 concentration of the
atmosphere at that time was a good 200 to 300 times the
concentration found today.”

200 to 300 times 1969’s CO2! Was Delvin’s source right? I think some life might have survived the first gassing. 🙂

High heat, humidity and CO2 and the plants ran wild and gorged themselves and when the food (CO2) ran out they left their bloated carcases to rot.
And with most of the CO2 sequestered in plant material the world cooled down a little. Not that it would need an excuse to cool down since it’s been doing that ever since it came together in a molten, bubbling blob.
Pure speculation or plausible description?

So now that CO2 has been proved to be the cause of global warming, we can use “the latest supercomputing technology” to determine how much CO2 was in the air back then and it was the same as today.

Steve, these people scare me and that is no joke. They never learned the scientific method. They have this wonderful tool, the computer, and an extraordinary communication infrastructure, and they produce this trash.

Children need to be taught logic. Kids that can’t hack logic should be steered from the sciences. If you want to stay in the sciences you should do science and you can’t if you don’t know what it is.

Could some of the Carboniferous plant explosion be due to increased atmospheric mixing in that age? In our time photosynthetic activity draws CO2 down to perhaps limiting levels during the day. Increased mixing would result in an environment where plant growth is less limited by CO2 than ours.

I can imagine heavy rain fall might have this effect.

OT – If the air held more moisture, would it result in a lowering of the sea level? Would the thermal expansion of water obviously offset this?

“High heat, humidity and CO2 and the plants ran wild and gorged themselves and when the food (CO2) ran out they left their bloated carcases to rot.
And with most of the CO2 sequestered in plant material the world cooled down a little”

Michael, all,
Estimates such as in the following article of the “efficiency” of “natural” carbon sequestration could be of interest.